1. Field of the Invention
This invention relates to skin-care products in general and specifically to sunscreen formulations comprising an emulsion of water and at least one oil. In particular, it relates to oil and water emulsions containing organic sunscreens or containing organic and/or inorganic (physical) sunscreen components. It further relates to suncare/skincare compositions capable of providing a high degree of protection from the harmful effects of ultraviolet radiation, such as sunburn and sun-induced premature aging.
2. Description of the Related Art
Ultraviolet radiation (UVR) is defined as radiation beyond the visible portion of the electromagnetic spectrum at its violet end. UVR consists of wavelengths from 200 to 400 nm and is subdivided into three bands from longer to shorter wavelengths as ultraviolet A (UVA), ultraviolet B (UVB), and ultraviolet C (UVC), respectively.
Human skin has a limited capacity to adapt to certain UV radiation when exposure to solar radiation is increased gradually. However, this protective mechanism fails when exposure increases abruptly. The sunburn response is generally associated with UVB exposure. Recent work, however has shown that large doses of UVA have detrimental effects on skin as well.
The damaging effects of sunlight on skin are well documented. The combination of a diminished ozone layer with the growing tendency for people to engage in outdoor activities is believed to increase the occurrence of skin cancer and also to accelerate premature aging of the skin.
As the body of evidence regarding the harmful effects of UV radiation grows, the cosmetic industry continues to formulate new products for providing enhanced protection against UVB and UVA radiation. Sunscreen agents or sunfilters are now incorporated into a variety of products for everyday use. These include moisturizers, creams, lotions, foundations, lipsticks, and other miscellaneous skin care products as well as shampoos and mousses. Such formulations are designed to at least partially protect human skin and hair from UV radiation.
The protective strength of a particular sunscreen agent on the skin depends on a variety of factors. Among these factors are distribution (or deployment) of the sunscreen molecules on the skin, the spectral UV properties of the sunscreen, the photostability of the sunscreen, the chemical structure, the concentration of the sunscreen, the penetration of the sunscreens into the stratum corneum, and the spreading properties of the vehicle and the subsequent adherence to skin.
Deployment of the sunscreen molecules over the surface of the skin determines to a major extent the protection delivered by various sunscreen formulations. Sunscreen formulations should be designed such that when applied to skin they deliver a film that covers both the peaks and the valleys of skin. Ideally, the sunscreen formulation can be applied to yield a film of uniform thickness on the skin with the sunscreen molecules homogeneously distributed within the film. The vehicle (the non-sunscreen component of the formulation) determines the manner in which the sunscreen molecules are deployed on the skin. In addition, the vehicle also controls to a large extent the ability of a sunscreen to protect skin after prolonged water exposure. Beyond protecting the skin from UV radiation, other formulation attributes such as product mildness and cosmetic elegance are also important to consumers.
The most common formulation type for topical sunscreens is an emulsion. Sunscreen products may be manufactured as either oil-in-water (O/W) or water-in-oil (W/O) emulsions. Consequently, it is important that the emulsification system be capable of creating stable emulsions with a variety of polar and non-polar sunscreen agents as well as cosmetic oils.